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The Colorado River is one of the principal rivers of the Southwestern United States and northern Mexico (the other being the Rio Grande). The 1,450-mile (2,330 km) Colorado River drains an expansive, arid watershed that encompasses parts of seven U.S. and two Mexican states. Rising in the central Rocky Mountains in the U.S., the river flows generally southwest across the Colorado Plateau and through the Grand Canyon before reaching Lake Mead on the Arizona–Nevada border, where it turns south toward the international border. After entering Mexico, the Colorado approaches the large Colorado River Delta at the tip of the Gulf of California between Baja California and Sonora. Known for its dramatic canyons and whitewater rapids, the Colorado is a vital source of water for agricultural and urban areas in the southwestern desert lands of North America. The river and its tributaries are controlled by an extensive system of dams, reservoirs, and aqueducts, which divert 90% of its water in the U.S. alone to furnish irrigation and municipal water supply for almost 40 million people both inside and outside the watershed.〔 The Colorado's large flow and steep gradient are used for generating hydroelectric power, and its major dams regulate peaking power demands in much of the Intermountain West. Intensive water consumption has dried the lower of the river such that it has not consistently reached the sea since the 1960s.〔 Beginning with small bands of nomadic hunter-gatherers, Native Americans have inhabited the Colorado River basin for at least 8,000 years. Between 2,000 and 1,000 years ago, the river and its tributaries fostered large agricultural civilizations, which may have been some of the most sophisticated indigenous cultures in North America. These societies are believed to have collapsed because of a combination of severe drought and poor land use practices. Most native peoples that inhabit the basin today are descended from other groups that settled in the region beginning about 1,000 years ago. Europeans first entered the Colorado Basin in the 16th century, when explorers from Spain began mapping and claiming the area, which later became part of Mexico upon its independence in 1821. Early contact between foreigners and natives was generally limited to the fur trade in the headwaters and sporadic trade interactions along the lower river. After the greater Colorado River basin became part of the U.S. in 1846, the bulk of the river's course was still largely the subject of myths and speculation. Several expeditions charted the Colorado in the mid-19th century, one of which, led by John Wesley Powell in 1869, was the first to run the rapids of the Grand Canyon. American explorers collected valuable information that would later be used to develop the river for navigation and water supply. Large-scale settlement of the lower basin began in the mid- to late-19th century, with steamboats providing transportation from the Gulf of California to landings along the Colorado River that linked to wagon roads into the interior of New Mexico Territory. Lesser numbers settled in the upper basin, which was the scene of major gold strikes in the 1860s and 1870s. Major engineering of the river basin began around the start of the 20th century, with many guidelines established in a series of domestic and international treaties known as the "Law of the River". The U.S. federal government was the main driving force behind the construction of hydraulic engineering projects in the river system, although many state and local water agencies were also involved. Most of the major dams in the river basin were built between 1910 and 1970, and the system keystone, Hoover Dam, was completed in 1935. The Colorado is now considered among the most controlled and litigated rivers in the world, with every drop of its water fully allocated. The damming and diversion of the Colorado River system have been opposed by the environmental movement in the American Southwest because of the detrimental effect on the ecology and natural beauty of the river and its tributaries. During the construction of Glen Canyon Dam, environmental organizations vowed to block any further development of the river, and a number of later dam and aqueduct proposals were defeated by citizen opposition. As demands for Colorado River water continue to rise, the level of human development and control of the river continues to generate controversy. ==Course== (詳細はLa Poudre Pass in the Southern Rocky Mountains of Colorado, at just under above sea level. After a short run south, the river turns west below Grand Lake, the largest natural lake in the state. For the first of its course, the Colorado carves its way through the mountainous Western Slope, a sparsely populated region defined by the portion of the state west of the Continental Divide. As it flows southwest, it gains strength from many small tributaries, as well as larger ones including the Blue, Eagle and Roaring Fork rivers. After passing through De Beque Canyon, the Colorado emerges from the Rockies into the Grand Valley, a major farming and ranching region where it meets one of its largest tributaries, the Gunnison River, at Grand Junction. Most of the upper river is a swift whitewater stream ranging from wide, the depth ranging from , with a few notable exceptions, such as the Blackrocks reach where the river is nearly deep. In a few areas, such as the marshy Kawuneeche Valley near the headwaters and the Grand Valley, it exhibits braided characteristics.〔 Arcing northwest, the Colorado begins to cut across the eponymous Colorado Plateau, a vast area of high desert centered at the Four Corners of the southwestern United States. Here, the climate becomes significantly drier than that in the Rocky Mountains, and the river becomes entrenched in progressively deeper gorges of bare rock, beginning with Ruby Canyon and then Westwater Canyon as it enters Utah, now once again heading southwest. Farther downstream it receives the Dolores River and defines the southern border of Arches National Park, before passing Moab and flowing through "The Portal", where it exits the Moab Valley between a pair of sandstone cliffs. In Utah, the Colorado flows primarily through the "slickrock" country, which is characterized by its narrow canyons and unique "folds" created by the tilting of sedimentary rock layers along faults. This is one of the most inaccessible regions of the continental United States. Below the confluence with the Green River, its largest tributary, in Canyonlands National Park, the Colorado enters Cataract Canyon, named for its dangerous rapids, and then Glen Canyon, known for its arches and erosion-sculpted Navajo sandstone formations. Here, the San Juan River, carrying runoff from the southern slope of Colorado's San Juan Mountains, joins the Colorado from the east. The Colorado then enters northern Arizona, where since the 1960s Glen Canyon Dam near Page has flooded the Glen Canyon reach of the river, forming Lake Powell for water supply and hydroelectricity generation. In Arizona, the river passes Lee's Ferry, an important crossing for early explorers and settlers and since the early 20th century the principal point where Colorado River flows are measured for apportionment to the seven U.S. and two Mexican states in the basin. Downstream, the river enters Marble Canyon, the beginning of the Grand Canyon, passing under the Navajo Bridges on a now southward course. Below the confluence with the Little Colorado River, the river swings west into Granite Gorge, the most dramatic portion of the Grand Canyon, where the river cuts up to into the Colorado Plateau, exposing some of the oldest visible rocks on Earth, dating as long ago as 2 billion years. The of the river that flow through the Grand Canyon are largely encompassed by Grand Canyon National Park and are known for their difficult whitewater, separated by pools that reach up to in depth. At the lower end of Grand Canyon, the Colorado widens into Lake Mead, the largest reservoir in the continental United States, formed by Hoover Dam on the border of Arizona and Nevada. Situated southeast of metropolitan Las Vegas, the dam is an integral component for management of the Colorado River, controlling floods and storing water for farms and cities in the lower Colorado River basin. Below the dam the river passes under the Mike O'Callaghan–Pat Tillman Memorial Bridge – which at nearly above the water is the highest concrete arch bridge in the Western Hemisphere – and then turns due south towards Mexico, defining the Arizona–Nevada and Arizona–California borders. After leaving the confines of the Black Canyon, the river emerges from the Colorado Plateau into the Lower Colorado River Valley (LCRV), a desert region dependent on irrigation agriculture and tourism and also home to several major Indian reservations. The river widens here to a broad, moderately deep waterway averaging wide and reaching up to across, with depths ranging from . Before channelization of the Colorado in the 20th century, the lower river was subject to frequent course changes caused by seasonal flow variations. Joseph C. Ives, who surveyed the lower river in 1861, wrote that "the shifting of the channel, the banks, the islands, the bars is so continual and rapid that a detailed description, derived from the experiences of one trip, would be found incorrect, not only during the subsequent year, but perhaps in the course of a week, or even a day." The LCRV is one of the most densely populated areas along the river, and there are numerous towns including Bullhead City, Arizona, Needles, California, and Lake Havasu City, Arizona. Here, many diversions draw from the river, providing water for both local uses and distant regions including the Salt River Valley of Arizona and metropolitan Southern California. The last major U.S. diversion is at Imperial Dam, where over 90 percent of the river's remaining flow is moved into the All-American Canal to irrigate California's Imperial Valley, the most productive winter agricultural region in the United States. Below Imperial Dam, only a small portion of the Colorado River makes it beyond Yuma, Arizona, and the confluence with the intermittent Gila River – which carries runoff from western New Mexico and most of Arizona – before defining about of the Mexico–United States border. At Morelos Dam, the entire remaining flow of the Colorado is diverted to irrigate the Mexicali Valley, among Mexico's most fertile agricultural lands. Below San Luis Río Colorado, the Colorado passes entirely into Mexico, defining the Baja California–Sonora border; in most years, the stretch of the Colorado between here and the Gulf of California is dry or a trickle formed by irrigation return flows. The Hardy River provides most of the flow into the Colorado River Delta, a vast alluvial floodplain covering about of northwestern Mexico. A large estuary is formed here before the Colorado empties into the Gulf about south of Yuma. Before 20th-century development dewatered the lower Colorado, a major tidal bore was present in the delta and estuary; the first historical record was made by the Croatian missionary in Spanish service Father Ferdinand Konščak on July 18, 1746. During spring tide conditions, the tidal bore – locally called ''El Burro'' – formed in the estuary about Montague Island in Baja California and propagated upstream. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Colorado River」の詳細全文を読む スポンサード リンク
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